The present invention is directed to a process for electrodepositing a two-coat composite coating on a substrate where the first electrodeposited coating protects against corrosion and the second electrodeposited coating protects against chipping of the coated composite.
Multilayered coating composites find use in various industries including the coating and/or painting of various motor vehicles. In several of these industries and in the automotive industry in particular there can be from 2 up to 6 or more coating layers in the multilayered coating composites. These coating layers can serve to protect the substrate and/or to provide a decorative finish.
Multilayered coating composites for metal substrates like those in the automobile industry have involved electrodeposition coatings as an initial resinous coating layer to protect the metal substrate from corrosion. Cationic electrodeposition coatings have become the coatings of choice for corrosion protection. Electrodeposition has become increasingly important in the coatings industry because by comparison with non-electrophoretic coating means, electrodeposition offers higher paint utilization, outstanding corrosion protection, low environmental contamination, and a highly automated process.
Two-coat application by the electrodeposition process is known in the art. For example in U.S. Pat. Nos. 4,988,420; 4,840,715; and 5,275,707 different types of electroconductive pigments are added to a first electrodeposited acrylic resinous coating, and a second electrodeposition coating is applied over the conductive first electrodeposited acrylic coating. Typically, these second electrodeposition coatings have been applied for durability and decorative purposes.
Also in multilayered coating composites for motor vehicles another coating layer that can be present is a spray applied chip resistant coating layer. Such a layer protects the surface of the substrates from losing paint through chipping when the substrate of the vehicle is hit with solid debris such as gravel and stones. The art for achieving chip resistance from spray applied primer coatings has postulated that reducing the differential in impact energy between multiple coating layers should improve chip resistance of the coating. This is especially the situation for those coating layers with excessive difference of hardness between them. This reduction in the differential would lessen delamination between the coatings such as between the undercoat and an intermediate coat and a top coat or an undercoat and an intermediate coat.
In U.S. Pat. No. 5,674,560, this differential is reduced through a chip resistant polyolefin type of primer that is spray applied over a cationic or anionic electrodeposited coated film before application of a soft intermediate polyester film. The reduction of the differential in impact energy is reportedly maximized when the polyolefin primer is applied over the softer anionic electrodeposited film as opposed to a cationic electrodeposited film.
Therefore, even though the art recognizes that cationic electrodeposited coatings provide better corrosion resistance than anionic electrodeposited coatings, further improvements in chip resistance in a multilayered coating system may be at odds with or involve sacrificing some corrosion resistance by using the anionic electrodeposited coating for corrosion protection.
An object of the present invention is to provide a process and system for the improved multiple coatings with both good chip resistance and corrosion protection while additionally providing efficiencies in application and processing. These include higher paint utilization, low environmental contamination, and a highly automated process.